It is well known that silver halide photographic elements can be spectrally sensitized to infrared radiation. See Mees and James, The Theory of Photographic Processes, 3rd edition, The MacMillan Company, 1966, pages 198 and 199. Silver halides are intrinsically sensitive only to light in the blue region of the spectrum. Therefore, when silver halides are to be exposed to other wavelengths of radiation, such as green light, red light or infrared radiation, a spectral sensitizing dye is required to render silver halide sensitive to such radiation. As known in the art, silver halides having adsorbed on the grains spectral sensitizing dyes can be made sensitive to radiation of a wavelength other than the intrinsic blue sensitivity.
With the advent of lasers, and particularly solid state laser diodes emitting in the infrared region of the spectrum (e.g. 750 to 1500 nm), the interest in infrared sensitization has greatly increased. Many different processes and articles useful for exposure to laser diodes have been proposed. These include C.A.T. (Computer Assisted Tomography) scanners, graphic arts products and infared sensitive false colour-sensitized photographic elements as described in U.S. Pat. No. 4,619,892. In this patent, photographic elements are described which are capable of providing full colour images without exposure to corresponding visible radiation. This false address elements comprise at least three silver halide emulsions layers on a substrate, each associated with different photographic color image forming materials and sensitized to three different portions of the spectrum with at least two layers sensitized to different regions of the infrared region of the spectrum.
There are numerous references to dye structures for infrared sensitizing dyes. Examples of patents disclosing such dyes are listed in U.S. Pat. No. 4,011,083. The most common infrared sensitizing dyes are tricarbocyanine dyes. "Tricarbocyanine" is a term used in the art to include dyes having an amidinium-ion chromophoric system (see Mees and James above, page 201). Typically, such dyes have two nitrogen containing heterocyclic nuclei which are joined by a straight chain methine linkage having seven methine groups. The carbon atom of the central methine group is referred to herein as the meso carbon atom of the methine linkage. Said methine chain is represented by the formula: ##STR2## wherein the carbon atom marked with the asterisk is the meso carbon atom. Tricarbocyanine infrared sensitizing dyes having a 4-quinoline nucleus are described in U.S. Pat. No. 4,536,473. Benzoxazole tricarbocyanine infrared sensitizing dyes having a halogen atom attached to the meso carbon atom are described in U.S. Pat. No. 4,959,294. Tricarbocyanine infrared sensitizing dyes containing an amino group attached to the meso carbon atom are described in U.S. Pat. Nos. 3,482,978 and 3,671,648. Tricarbocyanine infrared desensitizing dyes containing cyclic amino group (such as pyrrolidino, piperidino and morpholino) attached through the nitrogen atom to the meso carbon atom of the methine linkage of the dye are described in U.S. Pat. Nos. 3,506,655 and 3,552,974. Tricarbocyanine infrared sensitizing dyes having a piperazinyl group attached through the nitrogen atom to the meso carbon atom of the methine linkage of the dye are described in U.S. Pat. Nos. 3,623,881 and 3,758,461.
Many infrared sensitizing dyes described in the prior art, however, are of limited utility for use in silver halide photographic elements intended for exposure to infrared radiation. Some dyes, especially those having an amino group attached to the meso carbon atom of tricarbocyanine dyes, exhibit a large hyprochromic shift of the absorption maximum and a sensitization wavelength not deep enough in the infrared region. Some dyes exhibit a broad absorbance spectrum which gives to the silver halide photographic element unwanted sensitivity in regions of the spectrum other than that of the emission of laser diodes, resulting in poor colour separation between layers of elements having multiple infrared sensitive layers (such as those described in U.S. Pat. No. 4,619,892). Another problem with many known infrared sensitizing dyes is the poor stability of their solutions during keeping which renders them of limited utility in making photographic elements. U.S. Pat. No. 5,061,618 discloses infrared sensitizing dyes of improved performance, but advises against tricarbocyanine dyes having an heterocyclic ring attached to the meso carbon atom of the methine linkage directly through an heteroatom (such as a nitrogen atom).